1. Field of the Invention
The present invention relates to a load driving device capable of achieving, by using a single external control signal, both on/off control of the device itself and PWM (pulse width modulation) driving of a load. For example, the present invention relates to an LED (light emitting diode) driver IC supplying a drive current to an LED, and an illumination device and a display device using this.
2. Description of Related Art
FIG. 7 is a block diagram showing a conventional example of an LED driver IC. As shown in the figure, the conventional LED driver IC is configured such that an enabling signal EN for performing on/off control of the LED driver IC is PWM-driven, to thereby PWM-drive a drive current Io that is supplied to an LED from an LED driver section, and thus to control the brightness of an LED (i.e., control a mean value of the drive current Io).
In other words, the LED driver IC of this conventional example is configured such that an enabling signal EN for performing on/off control of the LED driver IC itself and a brightness control signal PWM for controlling the brightness of an LED by PWM driving are combined into a single external control signal.
An example of conventional technologies that the present invention pertains is disclosed, for example, in JP-A-2008-61482 (hereinafter, Patent Document 1).
It is true that, with the above described conventional LED driver IC, since it is possible to achieve, by using a single external control signal, both on/off control of the LED driver IC itself and brightness control of the LED by PWM driving, the number of pins can be reduced.
However, with the above-described conventional LED driver IC, since it is configured such that the brightness control of the LED is performed by turning on/off the LED driver IC itself, internal circuits integrated on the LED driver IC are all started over from an off state each time the enabling signal EN is raised from low to high level.
Thus, with the above-described conventional LED driver IC, between the time when the enable signal EN is raised to high level and the time when the drive current Io supplied to the LED rises up to a desired current value, there arises a time delay d (see FIG. 8) associated with start-up of a reference voltage supply, a reference current supply, and the like that form the LED driver section (in particular, rising-up of a reference voltage Vref).
This time delay d makes it impossible to obtain a drive current Io corresponding to the on-duty (ratio of a high level period with respect to a cycle T in PWM driving) of the enabling signal EN. As a result, with the above-described conventional LED driver IC, as indicated by a dash-dot line and a dash-dot-dot line in FIG. 9, a relation between the on-duty of the enabling signal EN and the drive current lo is more deviated from an ideal one (see a solid line in FIG. 9) with increase in frequency f (=1/T) of PWM driving, and this invites degradation in linearity, and reduction in dynamic range, of PWM driving.
Incidentally, the above problems can be solved with a configuration in which the enabling signal EN and the brightness control signal PWM are independently inputted, but, inconveniently, such a configuration requires an increased number of pins.
The above description deals with an LED driver IC as an example to describe the problems that the present invention intends to overcome, but these problems are common to load driving devices in general that achieve both on/off control of the devices themselves and PWM driving of loads by using a single external control signal.